Mg2+ is the most abundant intracellular divalent cation. Its chemistry is unique among the biological cations. Likewise, its various transport systems are also unique or are highly unusual members of existing families. Mg2+ also plays a fundamental regulatory role in prokaryotic cells and especially in bacterial pathogenesis. Our long-term goal is to understand both Mg2+ transport processes and Mg2+ regulation of gene expression to formulate an overall picture of Mg2+ homeostasis. CorA represents a new family of transport proteins lacking similarity to other known proteins. It is ubiquitous in the Bacteria and Archaea with homologs in yeast. Phylogenetic and structural studies suggest the various members of the family may differ in both function and membrane topology. All CorA's contain a large soluble N-terminal domain and a small C-terminal membrane domain, facilitating study of its structure through treatment as two separable structural domains. CorA has been shown to be oligomeric, the soluble domain has been purified and retains secondary structure, and a partial pathway for Mg2+ movement through the membrane has been identified. This renewal outlines continuing studies to define the functional structure of CorA. The 23 kDa MgtC protein is encoded by the first gene of a two gene operon along with the P-type ATPase Mg2+ transporter MgtB. Operon transcription is controlled by extracellular Mg2+ concentration through the PhoPQ two component regulatory system. PhoPQ is an essential regulator of virulence in S. typhimurium. Insertional mutagenesis of the mgtCB operon renders S. typhimurium essentially avirulent, with the large majority of this decrease in virulence due to loss of mgtC. Expression of MgtC is unusual. Transcription of the operon is increased several thousand fold at low extracellular Mg2+ concentrations, but only MgtB protein is expressed. In contrast, if the mgtB gene is insertionally inactivated, MgtC is expressed in large amounts. MgtC has been purified to homogeneity and appears to be an oligomeric membrane protein. Preliminary data suggest that it alters cellular Ca2+ homeostasis. This renewal proposal seeks to determine the basis for its unusual expression and to investigate its possible role in Ca2+ homeostasis.